The present invention relates to digital image processing.
FIG. 1A illustrates a digital image processing system. An image capture system 110 (e.g. part of a digital movie camera) captures an image of a scene 120 and generates digital data values G(x,y,t), where (x,y) are spacial coordinates of a picture element (pixel) and t is time. See FIG. 2. The image signal G(x,y,t) may contain noise caused by uneven numbers of photons captured by the system 110 (“photon noise”), imperfections of system 110, temperature variations, and possibly other sources. A digital image processing system 130 (e.g. part of a digital camera or a stand-alone computer), which includes a noise reduction module 134, performs some image improvements and outputs improved digital image data F(x,y,t). The data F(x,y,t) can be provided to an image display system 140 (e.g. a computer monitor or a television set) to display the image for viewing.
FIG. 1B illustrates the noise reduction module 134 in more detail. Module 134 receives a digital signal S(x,y,t) which is either equal to G(x,y,t) or obtained from the G signal via some processing. A spacial filter 210 performs some kind of spacial averaging for each fixed time t. Spacial filter examples are described in U.S. patent application Ser. No. 09/906,705 published as no. 2002/0028025 on Mar. 7, 2002, filed on Jul. 18, 2001 by Hong and incorporated herein by reference. The spacial filter's output signal Sxy(x,y,t) is processed by a temporal filter 220 to take advantage of the image correlation at a given pixel at different times t. The temporal filter output is shown as S*(x,y,t). The signal S*(x,y,t) can be provided as signal F(x,y,t) on the output of system 130 (FIG. 1A), or can be processed further to obtain the signal F(x,y,t).
Noise reduction operations such as performed by module 134 involve a trade-off between effective noise reduction on the one hand and edge and detail blurring on the other hand. Another trade-off is that good image improvement may involve much processing and hence require high-cost circuitry and/or a long processing time. Some spacial filters 210 of FIG. 1A generate the value Sxy(x,y,t) as a linear combination of the values S(x′,y′,t) with coefficients set to zero if the difference |S(x,y,t)−S(x′,y′,t)| exceeds a predefined threshold. Exceeding the threshold is taken as an indication of an edge or a detail in the image. The corresponding coefficients are zeroed out in order not to blur the edge or the detail.
It is desirable to provide new noise reduction systems which will be time and cost efficient and will provide effective noise reduction for at least some images.